In recent years, advances in biotechnology have led to an ability to produce eukaryotic polypeptides on an industrial scale by the expression of appropriate genes in the cells of host cell-lines. The commercial success of such processes depends upon the efficiency with which a particular gene can be expressed in the cells of a given host cell-line. There is a continued need for expression systems exhibiting high expression efficiency.
Early expression experiments were conducted using bacterial cell-lines. However, the disparity between such cellular environments and the normal environment of eukaryotic polypeptides leads to some undesirable effects and generally to a low product output. These effects have been, to some extent, ameliorated by the use of eukaryotic cell-lines, particularly mammalian cell-lines, as hosts. A variety of mammalian cell-line expression systems are known (see for example U.S. Pat. No. 4,419,446 (Howley, et al) which describes the transformation of mouse fibroblast cells with a vector including a gene coding for human insulin).
Studies relating to the expression of immunoglobulin genes have involved the use of recombinant DNA techniques to clone the relevant genes and to transform mammalian cells with the cloned genes. (Morrison, S. L. and Oi, V. T. Ann Rev. Immunol. (1984) 2 239-256). Myeloma cell-lines have been used as hosts and various vectors have been produced to transform myeloma cell-lines. These vectors in general carry an immunoglobulin promoter and gene. The vectors also include selectable markers to allow selection of host myeloma cells which have been successfully transformed. The markers comprise viral promoters driving expression of prokaryotic genes coding for non-secreted products which confer antibiotic resistance on transformed host cells. The products, which have a catalytic inactivating effect upon antibiotics, are produced at low levels.
These studies have shown that myeloma cell-lines exhibit a very specialised function in the expression of immunoglobulin genes. It has been shown that chimaeric mouse/human immunoglobulins can be produced in myeloma cells (Morrison, S. L. et al PNAS USA (1984) 81 6851-6588; Neuberger, M. S. Nature (1985) 314 268-270; Boulianne, G. L. Nature (1984) 312 643-646) and also that chimaeric immunoglobulin/enzyme and immunoglobulin/antigen polypeptides can be produced in myeloma cells. In all cases, an immunoglobulin promoter is used to direct expression of the desired product which comprises, at least in part, an immunoglobulin.
It has now been surprisingly discovered that it is possible to express eukaryotic genes at high levels in myeloma cell-lines from non-immunoglobulin promoters. This result is surprising in view of the specialised nature of myeloma cells and the known dependence upon cell type of the expression of eukaryotic genes.